Process for producing ingot

ABSTRACT

A large and uniform ingot is produced by casting smaller ingots from a melt of a metallic material which may include a high-melting-point active metal such as Ti or a Ti alloy obtained by a cold crucible induction melting method, forming a consumable electrode by using these smaller ingots, re-melting this consumable electrode by a vacuum arc re-melting method, and casting a larger ingot by using the obtained melt.

This application is a continuation of International Application No. PCT/JP2006/305099, filed Mar. 15, 2006 which claims priority on Japanese Patent Application 2005-169319 filed Jun. 9, 2005.

BACKGROUND OF THE INVENTION

This invention relates to a process for producing an ingot and more particularly to a process for economically producing a large and uniform ingot.

For mass-producing wrought products of Ti or Ti alloys, for example, large and uniform ingots are necessary on the order of several tons. This invention relates to a process for producing large and uniform ingots even in the case of an active metal having a high melting point such as Ti.

As an example of process for producing an ingot containing an active metal having a high melting point such as Ti, Japanese Patent Publication Tokkai 2001-131651 has disclosed a process of melting a metallic material by a cold crucible induction melting method and obtaining an ingot by pouring its melt into a mold. Japanese Patent Publication Tokkai 9-31558 has disclosed another process of obtaining an ingot by re-melting a consumable electrode made of a metallic material by a vacuum arc re-melting method and pouring its melt into a mold.

The prior art process of melting a metallic material by a cold crucible induction melting method is advantageous in that the melts become uniform as a whole since they are mixed together collectively. As a result, the overall composition of the melt can be grasped by sampling and analyzing only a portion of it and it can be adjusted, if it is different from its target value, by adding a necessary material thereto.

This process is disadvantageous, however, in that a large crucible with a power-supply equipment is necessary because a high-frequency current must be passed through the coils provided on its outer periphery. Such a crucible with a large capacity is difficult to construct, and since it would be a heavy financial burden to construct and operate an apparatus with a large crucible, an apparatus equipped with a significantly smaller crucible is usually constructed and operated, obtaining relatively smaller ingots. Smaller ingots are disadvantageous from the point of view of yield and productivity as they are used in a production process. In order to serve as wrought products, ingots of at least one ton are considered to be necessary.

The prior art process of producing an ingot by re-melting a consumable electrode made of a metallic material by a vacuum arc re-melting method is capable of producing large ingots of several tons. Since the consumable electrodes that are used by this method are produced by forming layers of a metallic material in the direction of its axis, its components in the direction of the axis become non-uniform and hence the ingot produced by using such a consumable electrode also becomes non-uniform.

It is therefore an object of this invention to provide a process for economically producing a large and uniform ingot even if the ingot contains an active metal with a high melting point such as Ti.

SUMMARY OF THE INVENTION

The present invention relates, in view of the object described above, to a process for producing an ingot comprising the steps of casting smaller ingots from a melt obtained by a cold crucible induction melting method, forming a consumable electrode by using these smaller ingots, re-melting this consumable electrode by a vacuum arc re-melting method to obtain another melt, and casting a larger ingot by using the latter melt.

By such a method, the composition of the (smaller) ingots cast by the cold crucible induction melting method can be adjusted within a target range and these ingots can be produced evenly. Thus, the distribution of components of the consumable electrode can also be made uniform in its axial direction, and a larger ingot which is uniform throughout can be produced if such a consumable electrode is re-melted by a vacuum arc re-melting method.

Cold crucible induction melting method and vacuum arc re-melting method of known types may be used according to this invention but a process (according to a first embodiment of the invention) by Steps A, B and C as described below is preferable, Step A comprising the step of melting a metallic material by a cold crucible induction melting method to obtain a melt and pouring this melt into a smaller mold to obtain a smaller ingot by casting, Step B comprising the step of joining a plurality of the smaller cast ingots after carrying out Step A once or more times, and Step C comprising the step of using the joined smaller ingots obtained in Step B as the consumable electrode for re-melting in a vacuum arc re-melting method to obtain the latter melt and obtaining the larger ingot by a casting method by pouring this latter melt into a larger mold larger than the aforementioned smaller mold.

According to a second embodiment of the invention, the cold crucible induction melting and vacuum arc re-melting methods are carried out by Steps a, b and c, Step a comprising the step of melting a metallic material by a cold crucible induction melting method to obtain a melt and pouring the melt into a portion of a larger mold to obtain a smaller ingot by casting, the smaller ingot being smaller than the space inside the larger mold, Step b comprising the step of joining a plurality of the smaller cast ingots inside the larger mold after carrying out Step a once or more times, and Step c comprising the step of using the joined smaller ingots obtained in Step b as a consumable electrode for re-melting by a vacuum arc re-melting method to obtain another melt and obtaining a larger ingot by a casting method by pouring the latter melt into the larger mold.

In the above, the smaller ingots mean smaller than the ingot obtained in Step C or c, and there is no particular limitation on their mass but their mass is usually intended to be on the order of several hundred kilograms to a ton. This is because the equipment to be used for the cold crucible induction melting method for obtaining ingots of this size is convenient from the operational and economical points of view. What is herein referred to as the larger ingot is usually of a mass on the order of several tons.

There is no particular limitation on the manner in which the smaller ingots are to be joined but a common method is to weld them together where they contact each other.

Neither is there any particular limitation on the shape of the smaller ingots to be sequentially formed and joined in a method according to the second embodiment of the invention. They may be sequentially formed or dummy partitions may be used and sequentially removed. In all cases, those ingots that are earlier hardened are integrated into those becoming hardened later.

Methods of joining the smaller ingots over surfaces extending along planes extending along the axis of the consumable electrode used in Step C or c are advantageous because the compositions of the ingots obtained in Steps A and a become uniform in the axial direction. Since a consumable electrode that is uniform in the axial direction can be obtained by joining them, a uniformly formed larger ingot can be easily produced by melting such a consumable electrode.

By these method, furthermore, the compositions of the ingots cast in Steps A and a may be different somewhat because the consumable electrode obtained by joining them together in Step B or b is melted again. Thus, it is sufficient if the composition of the mixed melts of these component ingots is within the allowable range of the target large ingot. If the first smaller ingot obtained contains too much of a certain element, for example, the content of that element may be reduced when the next smaller ingot is produced.

Methods of joining the smaller ingots over surfaces extending along planes perpendicular to the axis of the consumable electrode used in Step C or c in Step B or adding them sequentially along this axis in Step b are advantageous because a larger ingot with a uniform composition in the axial direction can be obtained without using a mold of any particular shape. When these methods are used to produce a large ingot, it is generally necessary to analyze samples of the melt in Step A or a and to use in Step B or b only those smaller ingots with components adjusted to be uniform. This method can be used in the cold crucible induction melting method because components can be grasped and adjusted.

This invention does not impose any limitation on the kind and composition of the metallic material to be used in Step A or a. This invention is particularly useful when a high-melting-point active metal such as Ti, Nb, W, Zr and Ta and their alloy is used, and more particularly when it is Ti or a Ti alloy. It is preferable in carrying out Steps A and a to sample a portion of the melt and to analyze it when the metallic material is melted and to readjust the composition of the melt based on the result of such analysis.

The present invention has the merit wherein a large and uniform ingot can be obtained economically even if a high-melting-point active metal such as Ti is contained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagonal view of a consumable electrode used in a method of this invention.

FIG. 2 is a diagonal view of another consumable electrode used in a method of this invention.

FIG. 3 is a diagonal view of still another consumable electrode used in a method of this invention.

FIG. 4 is a diagonal view of an example of the step of pouring a melt according to this invention.

FIG. 5 is a sectional view of an example of the step according to this invention subsequent to the step shown in FIG. 4.

FIG. 6 is a sectional view of an example of the step according to this invention subsequent to the step shown in FIG. 5.

FIG. 7 is a sectional view of an example of the step according to this invention subsequent to the step shown in FIG. 6.

FIG. 8 is a sectional view of an example of the step according to this invention subsequent to the step shown in FIG. 7.

FIG. 9 is a sectional view of an example of the step according to this invention subsequent to the step shown in FIG. 8.

FIG. 10 is a sectional view of an example of the step according to this invention subsequent to the step shown in FIG. 9.

FIG. 11 is a sectional view of an example of the step according to this invention subsequent to the step shown in FIG. 10.

FIG. 12 is a sectional view of an example of the step according to this invention subsequent to the step shown in FIG. 11.

FIG. 13 is a sectional view of an example of the step according to this invention subsequent to the step shown in FIG. 12.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagonal view of a consumable electrode used in one of the methods (the first method) of this invention. As shown, it is constructed by assembling four smaller ingots 1 a, 1 b, 1 c and 1 d obtained by cutting a cylinder with a pair of mutually perpendicular planes crossing each other along its axis and soldering them together so as to form a single consumable electrode 1.

FIG. 2 is a diagonal view of another consumable electrode used in the first method of this invention, being constructed by assembling four smaller ingots obtained by assembling four smaller ingots 2 a, 2 b, 2 c and 2 d by cutting a cylinder with three mutually parallel planes extending parallel to its axis and soldering them together so as to form another single consumable electrode 2.

FIG. 3 is a diagonal view of still another consumable electrode used in the first method of this invention, being constructed by assembling four smaller ingots obtained by assembling four smaller ingots 3 a, 3 b, 3 c and 3 d by cutting a cylinder with three mutually parallel planes extending radially and soldering them together so as to form still another single consumable electrode 3.

FIGS. 4-13 are drawings (FIG. 4 being a diagonal view and FIGS. 5-13 being transverse sectional views) for showing the casting routines in Step a and Step b of another method (the second method) of this invention. As shown in FIGS. 4 and 5, four frames 4 a, 4 b, 4 c and 4 d of the same shape are inserted slidingly into a larger mold 4 having a cylindrical inner space so as to be intimately in contact with one another. Next, one of these four frames (4 a) is removed, as shown in FIGS. 6 and 7, and a melt is poured into this empty space, which is left there by the removal of this frame 4 a, by the cold crucible induction melting method such that a smaller ingot 5 a is produced.

Next, as shown in FIGS. 8 and 9, another frame 4 b is removed and a melt is poured into the empty space left there in the shape of the frame 4 b by its removal such that not only another smaller ingot 5 b is produced by the cold crucible induction melting method but also that it will be joined to the earlier produced ingot 5 a to form a single integrated ingot.

Next, as shown in FIGS. 10 and 11, still another frame 4 c is removed and a melt is poured into the empty space left there in the shape of the frame 4 c by its removal such that not only still another smaller ingot 5 c is produced by the cold crucible induction melting method but also that it will be joined to the earlier produced ingots 5 a and 5 b to form a single integrated ingot.

Finally, thereafter, as shown in FIGS. 12 and 13, the fourth frame 4 d is removed and a melt is poured into the empty space left there in the shape of the frame 4 d by its removal such that not only still another smaller ingot 5 d is produced by the cold crucible induction melting method but also that it will be joined to the earlier produced ingots 5 a, 5 b and 5 c to form a single integrated larger ingot. This finally obtained larger ingot is used as a consumable electrode in a vacuum arc re-melting method which is herein referred to as Step c.

Although a method by using dummy frames has been described above with reference to FIGS. 5-13 as the second method, the second method according to this invention may be carried out alternatively by sequentially forming layers inside a mold by adding a melt. Such a method may be carried out by sequentially adding a melt in the axial direction inside a melt or by keeping the mold sideways in a laid-down position and adding a melt from above sequentially.

TEST EXAMPLE 1

Steps a and b were carried out as explained above with reference to FIGS. 4-13 by using a Ti alloy (6Al-4V-Ti) as metallic material and carrying out a cold crucible induction melting method under the following conditions: Inputted power: 3000 kW; Frequency: 2500 Hz; Inner diameter of crucible:  700 mm; Amount melted per cycle:  450 kg; Mass of each ingot:  400 kg.

Next, the larger joined ingot 5 obtained in Steps a and b was used as a consumable electrode to carry out a vacuum arc re-melting method by Step c under the following conditions: Current intensity: 15 kA; Voltage:  3 5V; Degree of vacuum:  1 Pa; Ratio of crucible inner diameter to outer diameter of consumable 1/0.85; electrode: Mass of larger cast ingot: 1.6 t. Sample pieces were scraped off from a top part, a middle part and a bottom part of the larger cast ingot and the contents (mass %) of Al and V were analyzed five times. The averaged values of these analyses are shown in Table 1 below.

COMPARISON EXAMPLE 1

Another consumable electrode was produced by press-molding a metallic material of the same Ti alloy as in Test Example 1 and used in a primary vacuum arc melting process under the same conditions as in Test Example 1 and the resultant melt was used to obtain a consumable electrode of the same size as in Test Example 1 for re-melting. The latter electrode was used to carry out a secondary vacuum arm melting (vacuum arc re-melting) under the same conditions as in Test Example 1 and an ingot of the same size as in Test Example 1 was cast from the melt thus obtained. Similar analyses were carried out on the ingot thus obtained and the obtained average values are also shown in Table 1 below. TABLE 1 Element Al (mass %) V (mass %) Position Top Middle Bottom Top Middle Bottom Standard 5.5-6.75 3.5-4.5 Aimed value 6.0 4.0 Test Example 1 6.05 6.01 6.03 4.02 4.03 3.99 Comparison 6.17 6.18 6.03 4.22 4.17 4.09 Example 1

Table 1 makes it clear that large and uniform ingots can be economically obtained by a method of this invention even if an active metal with high melting point such as Ti is contained. Although results of experiments using the latter of the two methods described are shown above, similar results were also obtained by experiments using the former method. 

1. A process for producing an ingot, said process comprising the steps of: casting smaller ingots from a melt obtained by a cold crucible induction melting method; forming a consumable electrode by using said smaller ingots; re-melting said consumable electrode by a vacuum arc re-melting method to obtain another melt; and casting a larger ingot larger than said smaller ingots by using said another melt.
 2. The process of claim 1 comprising Step A, Step B and Step C wherein said Step A comprises melting a metallic material by a cold crucible induction melting method to obtain said melt and pouring said melt into a smaller mold to obtain smaller ingots by casting; wherein said Step B comprises joining a plurality of the smaller cast ingots after carrying out said Step A once or more times; and wherein said Step C comprises using the joined smaller ingots obtained in said Step B as said consumable electrode for re-melting to obtain said another melt and obtaining said larger ingot by a casting method by pouring said another melt into a larger mold larger than said smaller mold.
 3. The process of claim 1 comprising Step a, Step b and Step c wherein said Step a comprises melting a metallic material by a cold crucible induction melting method to obtain said melt and pouring said melt into a portion of a larger mold to obtain a smaller ingot by casting, said smaller ingot being smaller than the space inside said larger mold; wherein said Step b comprises joining a plurality of the smaller cast ingots inside said larger mold after carrying out said Step a once or more times; and wherein said Step c comprises using the joined smaller ingots obtained in said Step b as said consumable electrode for re-melting to obtain said another melt and obtaining said larger ingot by a casting method by pouring said another melt into said larger mold.
 4. The process of claim 2 wherein said Step B comprises joining said plurality of the smaller cast ingots over surfaces extending along the axis of the consumable electrode used in said Step C.
 5. The process of claim 3 wherein said Step b comprises joining said plurality of smaller cast ingots sequentially over surfaces extending along the axis of the consumable electrode used in said Step c.
 6. The process of claim 2 wherein said Step B comprises joining said plurality of the smaller cast ingots over surfaces extending perpendicularly to the axis of the consumable electrode used in said Step C.
 7. The process of claim 3 wherein said Step b comprises joining said plurality of smaller cast ingots sequentially over surfaces extending perpendicularly to the axis of the consumable electrode used in said Step c.
 8. The process of claim 2 wherein said metallic material is a high-melting-point active metal or an alloy thereof.
 9. The process of claim 3 wherein said metallic material is a high-melting-point active metal or an alloy thereof.
 10. The process of claim 2 wherein said metallic material is Ti or an alloy thereof.
 11. The process of claim 3 wherein said metallic material is Ti or an alloy thereof. 